In order to realize the full potential of magnetic random access memory (RAM), whether based on tunnel magnetoresistance (TMR) or giant magnetoresistance (GMR), fundamental challenges at the basic cell design need to be addressed. Such challenges include (1) scalability (e.g., decreasing drive currents and stable error rates with decreasing feature size); (2) endurance (e.g., the number of read/write cycles before cell breakdown); and (3) thermal stability of stored information (e.g., stability against errors due to thermally-induced transitions between two states that represent different bit values, an effect that increases with decreasing element volume and comes into play at deep nanoscale feature sizes).
Cell design features that we have conceived to enable scalability, increased endurance, and thermal stability include (1) a closed-flux cell structure, (2) parallel drive lines at the memory cell, and (3) increased film thickness. Each of these design features is described in more detail below.
An issue that transcends the individual cell design is the compatibility of magnetic RAM fabrication technology with CMOS processing. Some success with regard to this issue has already been demonstrated by commercial magnetic RAM. Another higher level issue is capacity (e.g., sufficient write and read margins for large arrays). We found that the issues attendant to scalability and capacity can be treated as distinct.
The issue of thermal stability has been resolved conceptually. See, for example, U.S. Pat. No. 7,911,830 entitled Scalable Nonvolatile Memory issued Mar. 22, 2011, the entire disclosure of which is incorporated herein by reference for all purposes.
We determined that a critical factor for scalability is control of the demagnetizing field Hd, i.e., the field produced by the magnetization M according to: V·Hd=−4π∇·M. The presence of Hd in a magnetic RAM (e.g., due to incomplete flux closure in its memory cells) causes multiple problems. Inside the memory cell the write current needs to overcome Hd to impress a given magnetization on the material, i.e., the write current must increase to write the bit value. Additionally, Hd from one cell can disturb the magnetization (i.e., change the bit values) of neighboring cells, causing errors. Increasing cell footprint can mitigate increase in error rates, but this sacrifices cell density. Hd also causes “shearing,” a decrease in the slope of the intrinsic hysteresis loop of a cell, that produces skewed minor loop operation and a resulting decrease in the signal strength of the cell's read signal. Moreover, in a cell without fully-closed flux, Hd increases strongly as feature size decreases into nanoscale.
These problems—increasing drive currents, increasing error rates, increasing cell footprint, and decreasing signal strength—are exacerbated by increasing demagnetizing fields that accompany decreasing feature size. These problems are further exacerbated by the interplay between thermal fluctuations and Hd in that the two can reinforce each other in specific configurations. To address these problems, we designed a memory cell that has a fully closed-flux and which is characterized by decisive advantages in power consumption, error rates, and memory density over designs with incomplete flux closure.
We also developed a measurement protocol to separate out the effects of demagnetizing fields from possible thermal effects in a magnetic RAM (based on either GMR or TMR) and to determine the magnitude of both. See R. Spitzer and E. Wuori, Demagnetizing Fields in Magnetic RAM, Intermag 2009, Session ET-06, the entire disclosure of which is incorporated herein by reference for all purposes.
Capacity is linked to the signal strength provided by the film (GMR or TMR) used in the memory cell. We chose GMR films for our cell design, despite the smaller signal of presently available GMR structures than that of TMR, for three reasons: (i) simplicity of cell design—the GMR cell size is about one-half that of the TMR cell size, and requires fewer than one-half the number of masking steps; (ii) the method for addressing thermal stability at deep nanoscale lends itself much more readily to GMR than to TMR structures; (iii) the functional memory components of our magnetic RAM—the memory array without support electronics—may be constructed with metals and insulators alone (no semiconductors). This provides the potential for monolithic 3D structures (vertically replicated 2D arrays). The storage density of such a 3D SpinRAM with 4 levels of 2D arrays can exceed that of a hard disk at 30 nm feature size and, for many mainstream applications (e.g., ones that depend on a specific number of input/output operations per second) it will likely be strongly economically competitive with hard disks. Additional information and examples are provided in U.S. Pat. No. 6,992,919 entitled All-Metal Three-Dimensional Circuits and Memories issued Jan. 31, 2006, the entire disclosure of which is incorporated herein by reference for all purposes.
To realize the strong sense signal needed for high capacity, we developed a ferromagnetically-coupled GMR superlattice with low drive fields and potential for significantly higher useful GMR values than currently available, well upwards of 50%. Examples of such a superlattice structure are described in U.S. Pat. No. 8,619,467 entitled High GMR Structure With Low Drive Fields issued Dec. 31, 2013, the entire disclosure of which is incorporated herein by reference for all purposes.
The issue of cell endurance is addressed by our crosspoint magnetic RAM with a coincident-current architecture and tied to the configuration of the drive lines in the memory array as described, for example, in U.S. Pat. No. 7,911,830 incorporated herein by reference above.
Despite these successes in the advancement of magnetic RAM design, further improvement continues to be our goal.